1. Field of the Invention
This invention relates to a control system for an air intake system for use with or in combination with a supercharged engine, and, more particularly, to an air intake control system for controlling air intake when cranking a supercharged engine.
2. Description of the Related Art
Typically, in order to start a supercharged engine, an air intake control system is used. When cranking a supercharged engine, an air intake system opens an air bypass passage so as to force intake air to bypass a mechanical supercharger 3 (see FIG. 1 for reference only) which is suspended in operation. The air introduced into the bypass passage is directed into cylinders 9 of the engine passing through surge tanks 7 and intake valves 8. This is because, during starting or cranking of the engine with the use of a starter motor, a charging efficiency is more advantageously increased with naturally aspirated air than with the use of the supercharger and, in addition, no load is added to the starter motor from the supercharger.
A supercharging technique, which improves thermal efficiency, and remarkably improves fuel consumption, is described in Japanese Unexamined Patent Publication No.63- 239312. Specifically, a supercharged engine described in the above mentioned publication has a geometric compression ratio established at a relatively high ratio of 8.5 or greater and a closing time of an intake valve retarded 50 degrees or greater after the bottom dead center. This supercharged engine can obtain a high expansion ratio while it can appropriately suppress the effective compression ratio of the engine, so that it provides a high resistance to knocking in a high load, high speed range of engine operation in which a large amount of air is supercharged. Further, in the supercharged engine, a drop in temperature of a fuel mixture caused by adiabatic expansion is considerably more as compared with a rise in temperature due to adiabatic compression, so that the temperature of exhaust gas can be dropped without enriching the fuel mixture, thereby improving thermal efficiency and fuel consumption.
However, if a mechanically supercharged engine, in which intake valves close at a timing retarded 50 degrees or more after the bottom dead center, is started while air is naturally aspirated, the air charging efficiency is greatly diminished by means of blowing back of intake air. As a consequence, an insufficient amount of engine torque is caused during the cranking of the engine, so as to render the engine difficult to start because the supercharger has a high mechanical resistance to the engine, in particular, while the engine is still cold.